Strength and Absorbed Energy in Instrumented Impact Tests of Al2O3

By use of strain-gage instrumentation to record impact loads as a function of time, the flexural strength at high strain rates and the energy absorbed in Charpy impact fracture of an alumina were measured. The energy absorption by Al₂O₃ impact specimens is shown to result principally from elastic mechanical loading. Extraneous energy losses in the Charpy test can easily exceed the magnitude of this mechanical specimen-loading energy and are shown to be inherent sources of error in impact testing of brittle materials.     

On Modulus and Fracture Toughness of Rigid Particulate Filled High Density Polyethylene

A Kaolin-filled, high-density polyethylene (HDPE) system was used to illustrate the influence of particulates on modulus and toughness of the bulk material. A variation of filler content, particulate size and coupling quality for two HDPE-matrix systems with different viscosity led to a strong dependency of elastic modulus and fracture toughness under various testing conditions, e.g. static loading, fatigue and impact.

A stiffness improvement with increasing filler content was achieved by all coupling qualities. The developed Kaolin reinforcement of HDPE with optimised coupling offers an improvement of the stiffness and toughness under all investigated loading conditions. The degree of improvement depends on the particulate size and matrix viscosity. The energy dissipation mechanisms were investigated by fractographic analysis.     

The hygrothermal effect on the fatigue behavior of carbon fiber/ peek laminate composites (I)

The hygrothermal effects on the fatigue behavior of the Carbon/PEEK laminated composites before and after impact damage were examined in this study. The [0/45/90/-45]_2s AS-4/PEEK laminated composites were immersed in 80°C hot water for 45, 90 and 200 days,and subjected to falling weight impact with an energy of 8.58 J and then immersed in 80°C hot water for 45 days. It was found that the tensile strength of AS-4/PEEK laminated composites decreased with the increase of exposure period. The injured AS-4/PEEK composites were subjected to a static load and a tensiontension fatigue load at various levels of stress amplitudes. The effect of stress amplitude on the fatigue life was studied. The experimental fatigue life under different stress amplitude tests were estimated by the median rank order statistic cumulative distribution function. Then,the fitting curves for estimated data were analyzed by the Weibull distribution function. The S-N curves for a series of cyclic loads at various survival probabilities were presented. The damage behaviors of composites after fatigue load test were also investigated by scanning electron microscope(SEM). Results indicated the fatigue lives of immersed specimens were shorter than those without hygrothermal effect, the impact damage affects the fatigue life of composite significantly.     

聚氨酯涂层加固混凝土梁的力学性能研究

通过对混凝土试件进行试验研究,在应变率为0.000 3 s^-1和0.065 s^-1的加载条件下进行三点弯曲试验,模拟静态荷载和冲击荷载,评估聚氨酯(PU)涂层在静态荷载和冲击荷载作用下增强混凝土试件力学响应的有效性。结果表明,PU涂层(在混凝土试件的正面或反面)的应用对提高混凝土结构的延性具有积极作用,最大应变和累积应变能随着PU涂层厚度的增加近似线性增加,反面涂覆的方法在静态荷载作用下对混凝土结构的加固效率更高,采用正面涂覆的方法可增强混凝土结构的抗冲击性,通过增加涂层厚度能有效提高加固能力。PU涂层在试件的最终破坏过程中不会脱胶,这意味着材料具有良好的附着力,能有效减弱试件的破碎效应。     

Failure load prediction of structural adhesive joints : Part 2: Experimental study

The objective of this study was to determine how the fracture of adhesive joints depends on elastic beam parameters describing the adherends and the applied loads. The basic specimen geometry was the cracked lap shear joint constructed of aluminium alloy with various adherend and bondline thicknesses. Loads were applied in different combinations of bending, tension and shear to generate a failure envelope for each adhesive and specimen geometry. It was found that crack propagation for precracked specimens occured at a critical strain energy release rate but was also a function of the G_I/G_II ratio and the bondline thickness. The experiments also showed that the loads required to propagate a crack in a precracked specimen were always lower than the loads required to break the fillet. Hence, by treating uncracked joints as being cracked, where the fictitious crack tip is assumed to coincide with the location of the fillet, a conservative estimate of the failure load is obtained.     

Impact fatigue of a polycarbonate/acrylonitrile-butadiene-styrene blend

This study analyzes the impact properties of a polycarbonate/acrylanitrile-butadiene-styrene (PC/ABS) blend. The specimens were prepared under various injection molding conditions, including filling time, melting temperature, and mold temperature. Impact tests were performed with a Dynatup drop weight impact tester at different impact energies (10, 15, 20, 25 J). The fracture mechanism was examined with a scanning electron microscopy. The results indicated that the load-time history of the PC/ABS blend has approximately a sinusoidal form in impact. The best injection molding conditions are a filling time of 12 s, a melting temperature of 260°C and a mold temperature of 80°C. In this case, the specimen shows the highest energy absorbed in single impact, together with the highest impact number in impact fatigue. The impact number and the accumulation energy seem to follow an exponential curve as the impact energy decreases. The PC/ABS blend material clearly exhibited ductile fracture with a continuous reduction in strength by viscoplastic deformation. The higher the impact number, the higher the accumulation energy. The accumulation energy of impact fatigue with impact energy 10 J is about 35–45 times greater than the energy absorbed in single impact. Tearing, shear fracture, and plastic deformation are the major fracture mechanisms of the PC/ABS blend matrix in single impact and repeated impact conditions.     

The determination of the elastic properties of an anisotropic polycrystalline graphite using neutron diffraction and ultrasonic measurements

The elastic properties of an extruded graphite (GR-280) used in nuclear industry have been examined. The lattice preferred orientation was determined by time-of-flight neutron diffraction that revealed weak texture with a texture index of less than 1.2. The bulk elastic properties of polycrystalline graphite with such a texture have been calculated using various averaging methods and compared with the properties obtained from the measurements of the longitudinal sound velocities, performed using special equipment at different hydrostatic pressures up to 150 MPa. The static elastic modulus of the GR-280 graphite as well as the diffraction elastic modulus was measured in situ by high resolution neutron diffraction by observing the shift of the (0 0 2) Bragg reflection under uniaxial loads up to 20 MPa. The static elastic moduli of two pyrolytic graphites have also been measured for comparison. It was shown that the anisotropy of the elastic properties of reactor graphite GR-280 is due to the crystallographic texture formed during the extrusion process, but the internal pores and microcracks are not closed even at a pressure of 150 MPa and they greatly influence the exact values of the bulk elastic moduli of graphite.     

Breaking energies of a nylon series subjected to high-speed transverse impact

In a study of the transient behavior of a series of nylon 6/6 yarns differing systematically in mechanical properties, the effects of high-speed, transverse missile impact upon yarn specimens were observed by high-speed photography. The loss in missile kinetic energy was determined directly from the reduction in missile velocity and was studied as a function of yarn tenacity and missile impact velocity. The shape of the missile energy loss curves was due to the partition of missile energy into yarn kinetic energy and yarn strain energy. The missile energy losses and yarn dynamic breaking strains were compared to static breaking energies and breaking strains for these yarns. The observed trends are discussed in terms of the differing yarn tenacities and test rates.     

Finite element stress analysis and strength evaluation of epoxy-steel cylinders subjected to impact push-off loads

The stress wave propagations and stress distributions in epoxy-steel cylinders in which the outside surface of a solid cylinder (steel) is adhered to the inside surface of a hollow cylinder (epoxy resin) subjected to impact push-off loads were analyzed using the finite element method (FEM). The impact push-off loads were applied to epoxy-steel cylinders on a solid cylinder by dropping a weight. The FEM code employed was ANSYS/LS-DYNA. It was found that the maximum principal stress occurs at the upper edge of the interface, where the rupture initiates in epoxy-steel cylinders under the impact push-off loads. Besides, it was also found that the normal stress near the upper edge of the interface increases as the rigidity and the initial impact velocity increase; meanwhile it decreases as the diameter and the height of the solid cylinder increase. The strength of epoxy-steel cylinders increases as the rigidity of the solid cylinder increases, and the diameter and the height of the solid cylinder decrease. In addition, it was observed that the characteristics of the joints subjected to the impact push-off loads are opposite to those of the joints subjected to the static push-off loads. Furthermore, experiments were carried out to measure the strain response of epoxy-steel cylinders subjected to impact and static push-off loads. Fairly good agreements were observed between the numerical and the measured results.     

High-Temperature Crack Growth in Monolithic and SiCw-Reinforced Silicon Nitride under Static and Cyclic Loads

Experimental results are presented on subcritical crack growth under sustained and cyclic loads in a HIPed Si₃N₄ at 1450°C and a hot–pressed Si₃N₄–10 vol% SiC_w composite in the temperature range 1300°–1400°C. Static and cyclic crack growth rates are obtained from the threshold for the onset of stable fracture with different cyclic frequencies and load ratios. Fatigue crack growth rates for both the monolithic and SiC_w-reinforced Si₃N₄ are generally higher than the crack growth velocities predicted using static crack growth data. However, the threshold stress intensity factor ranges for the onset of crack growth are always higher under cyclic loads than for sustained load fracture. Electron microscopy of crack wake contact and crack–tip damage illustrate the mechanisms of subcritical crack growth under static and cyclic loading. Critical experiments have been conducted systematically to measure the fracture initiation toughness at room temperature, after advancing the crack subcritically by a controlled amount under static or cyclic loads at elevated temperatures. Results of these experiments quantify the extent of degradation in crack–wake bridging due to cyclically varying loads. The effects of preexisting glass phase on elevated temperature fatigue and fracture are examined, and the creep crack growth behavior of Si₃N₄–based ceramics is compared with that of oxide-based ceramics.     

Comparisons of Static and Impact Loading Damage in Zinc Sulfide

Contact damage was induced in CVD zinc sulfide by static and impact loading using glass spheres. Calculations based on the relation between the residual depth of the indentation and the ratio of the hardness to Young's modulus were used to estimate the dynamic hardness and impact load. Comparisons of applied loads, threshold loads for crack formation, and crack lengths for static and impact indentations at equal indentation radii show that predictions of impact damage using data from static loading are inadequate.     

Fracture toughness of phenolphthalein polyether ketone

The static and impact fracture toughness of phenolphthalein polyether ketone (PEK-C) were studied at different temperatures. The static fracture toughness of PEK-C was evaluated via the linear elastic fracture mechanics (LEFM) and the J-integral analysis. Impact fracture toughness was also analyzed using the LEFM approach. Temperature and strain rate effects on the fracture toughness were also studied. The enhancement in static fracture toughness at 70°C was thought to be caused by plastic crack tip blunting. The increase in impact fracture toughness with temperature was attributed two different mechanisms, namely, the relaxation process in a relatively low temperature and thermal blunting of the crack tip at higher temperature. The temperature-dependent fracture toughness data obtained in static tests could be horizontally shifted to match roughly the data for impact tests, indicating the existence of a time–temperature equivalence relationship. © 1995 John Wiley & Sons, Inc.     

Static and Cyclic Fatigue of Alumina at High Temperatures: II, Failure Analysis

Failure mechanisms of an alumina, tested at 1200°C under static and various cyclic loading conditions, were examined. Slow crack growth of a single crack is the dominant mechanism for the failure in specimens under cyclic loading with a short duration of maximum stress at all applied stress levels, as well as at high applied loads for static loading and cyclic loading with a longer hold time at maximum stress. At low stress levels, failure of static loading and cyclic loading with a longer hold time at maximum stress might occur by formation and/or growth of multiple macrocracks. More importantly, for all the given loading conditions. The viscous glassy phase behind the crack tip could have a bridging effect on the crack surfaces. A simplified model for calculating effective stress intensity factor at the crack tip under static and various cyclic loading demonstrated a trend consistent with the stress–life data.     

Fracture mechanics and relaxation processes in polyvinyl chloride

Linear elastic fracture mechanics has been applied to investigate the strain rate sensitivity of polyvinyl chloride over a range of temperatures (?197 to +80°C). Three-point bend tests have been carried out at two widely different strain rates; in “static” (slow bend) conditions and in a “dynamic” (impact) situation. A specially instrumented Charpy impact hammer has been used to analyse the record of transient loads. The fracture toughness results show a close relationship to the relaxation phenomena between ?60 and +75 °C; it was observed that in this region the maximum toughness correlated with the maximum β-relaxation. It was also found that, while in the region of low temperatures the material showed a considerable degree of strain rate sensitivity, at +50° the dynamic and static toughness were of the same order of magnitude; this behavior differs substantially from that observed in metals.     

温度对芳纶/玻璃纤维复合材料层压板冲击性能的影响

 论证了芳纶/玻璃纤维复合材料层压板在温度变化下的耐冲击载荷的试验结果。研究了在-50~120℃的温度范 围内,几个低速冲击能(8J,15J和25J)作用下,温度对层压板最大能量、弹性能、最大挠度的影响。结果表明,在 研究的温度范围内,复合材料的冲击性能受到影响。     

Discrete element method models of elastic and elastoplastic fiber assemblies

Geometry-dependency and plasticity are considered in the contact force models for the discrete element method simulations of elastic and elastoplastic fiber assemblies subject to uniaxial compression. It is observed that the contact force models have a significant impact on compressive loads. A simplified normal contact force model leads to smaller loads at large solid volume fractions compared to the experimental results, while the present geometry-dependent models give better predictions. Simulations with a simple Coulombic tangential contact force model (considering sliding friction only) significantly underestimate the loads, while a Mindlin tangential force model that considers static friction improves the predictions. For the modeling of the fibers that undergo large plastic deformations, plasticity should be considered for both the fiber bending and fiber–fiber normal contact in order to obtain correct simulation results. Elastic models for fiber–fiber contact and fiber bending deformation remarkably overpredict the loads for the plastic fibers.     

Damage characterization of repeatedly impacted glass fiber reinforced polyester‐armor steel composites with cone beam computed tomography technique

This article utilizes the characterization of single and repeated low velocity impact damage behavior of glass fiber reinforced polyester (GFRP) armor steel composite. Cone beam computed tomography technique (CBCT) was used for damage assessment. Impact energies, maximum loads and the permanent deflection of GFRP, armor steel composites are determined with instrumented drop weight impact test machine. The repeated impact performance and damage resistance were evaluated. On the other hand, preliminary single impact loading tests also performed in order to find the energy levels, which were ranged fully elastic energy level to perforation energy level for GFRP, armor steel composites. Additionally, CBCT was used to provide a novel, multiscale approach for assessing impact damage. Deformation areas of both single and repeatedly impacted GFRP, armor steel composites were assessed three‐dimensionally by CBCT. An innovative approach was used to visualize the internal damages. POLYM. COMPOS., 37:583–593, 2016. © 2014 Society of Plastics Engineers     

Influence of chemical structure of hardener on mechanical and adhesive properties of epoxy polymers

The mechanical and adhesive properties of epoxy formulations based on diglycidyl ether of bisphenol A cured with various aliphatic amines were evaluated in the glass state. Impact and uniaxial compression tests were used to determine the impact energy, elastic modulus and yield stress, respectively. The adhesion tests were carried out in steel–steel joints using single‐lap shear, T‐peel, and impact adhesive joints geometry. The better mechanical and adhesive behavior of the networks is obtained when exists high flexibility of chain between crosslink and/or high elastic modulus. The 1‐(2‐aminoethyl)piperazine epoxy network presents the best adhesive properties, high flexibility, and the largest impact energy. However, it possesses low elastic modulus and yield stress. Also, exhibits increases in peel strength and impact energy while reductions in lap shear strength. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

疲劳/蠕变交互作用下塑木复合材料的断裂损伤

塑木复合材料在动态载荷作用下，其断裂损伤并非纯疲劳或纯蠕变作用的结果。利用交变载荷的试验方法，研究了在疲劳／蠕变交互作用下塑木复合材料的断裂损伤行为。结果表明，在交变载荷为破坏载荷的80％和60％时，其疲劳／蠕变断裂曲线为三段式曲线，即瞬时弹性变形阶段、延迟弹性变形阶段和加速断裂阶段；在交变载荷为破坏载荷的40％时，38h内其疲劳／蠕变曲线为两段式曲线。随着最大载荷保持时间的增加，塑木复合材料进入延迟弹性变形阶段越晚，弯曲挠度增加越快，断裂寿命降低。     

Effect of loading rate on damage of concrete

In this study, damage mechanics theory was used directly to determine the damage of concrete subjected to both static and impact compressive loadings. Two approaches based on the variations of (1) elastic modulus (E) and (2) strain rate (ε), were used. Results from both approaches indicated that the damage to concrete at peak load depended mainly on the rate of loading, as it increased with increasing loading rate. The specimens subjected to impact loading were found to suffer higher damage than those subjected to static loading, as seen by the larger value of D at peak load (0.8-0.9 for concrete subjected to impact loading and 0.65 for concrete subjected to static loading). Beyond the peak, the strain energy was sufficient to cause the damage to increase to one (D=1) without any further applied load.